Richard Emslie of IUCN's Species Survival Commission told BBC News that a trio of experts systematically scoured 1,200 miles (2,500 kilometers) of habitat in northern Cameroon.

The survey failed to find any sign of the West African black rhino.

"They looked for spoor [tracks or droppings], they looked for the rhino's characteristic way of feeding, which has an effect like a pruning shear," Emslie, a rhino expert based in KwaZulu-Natal Province, South Africa, told the news service.

"But they didn't find anything to indicate a continued presence in the area.

"They did, however, come across lots of evidence of poaching, and that's the disconcerting thing."

Poachers have hunted the animals for decades for their horns to supply markets in Yemen and Asia (wallpaper: guarding a black rhino).